The cord construction of a tensile strand is typically such that a number of filaments or fibers form a yarn and a group of one or more yarns form a so-called ply, and in turn a group of plies form a cord.
Drive belts may be constructed as flat belts, v-belts, v-ribbed belts, toothed belts and clutch belts. The v-ribbed belt (VRB) is of particular significance in the automotive sector, its primary function in vehicles being to drive auxiliary assemblies, as for instance to drive a cooling water pump of an air-conditioning compressor or an alternator. V-ribbed belts, as will be appreciated, are also used in customary domestic appliances, for example washing machines.
Yarn twist and cord twist refer respectively to the number of turns applied per meter to the fiber ensemble of a yarn and the folding twist applied to the plies as they are being twisted together to form a cord. This twist is needed to confer coherency on respectively the fiber ensemble and the ensemble of plies and to improve flex fatigue resistance. Twist may have an S-direction or a Z-direction.
The number of turns per meter (t/m) is determined per meter of length. The designation of a yarn as “300 Z”, for example, is to be understood as meaning that the filaments of the yarn were twisted/spun at 300 turns per meter in the Z-direction.
V-ribbed belts in the automotive sector as well as most v-belts utilize mainly twisted cords of polyester as tensile strands. These cords are constructed from three, occasionally even from four or five twisted plies. The plies are first individually twisted from yarns consisting of filaments or fibers. This is the so-called “first twist”. In a second step, the plies are gathered together and twisted into a cord in a direction contrary to the direction of twist of the plies. This is the final twist. If, therefore, the plies have been twisted in the S-direction, the cords will be twisted in the Z-direction. This balances the torsional force which remains in the elements as a result of twisting.
Usually, the number of turns (t/m) chosen for the first twist will be somewhat higher than for the final twist. Twisting the plies and the cords is a costly manufacturing step. The greater the number of plies making up a cord, the greater the amount of twisting that is required.
US 2012/0115658 A1, which is incorporated herein by reference, discloses a drive belt having a PET tensile strand having a linear density of more than 3600 dtex, for which 2-ply cords are also mentioned as a possibility. The first twist is done at ≥180 t/m and the final twist at ≤160 t/m. The power range in which such cords of low linear density are employable is but limited. They are overtaxed at a high rate of power transfer, for example to a comparatively large generator, the belts might fail as a result of cord uncoiling or as a result of excessive lengthening. In addition, twisting to high numbers of turns is costly.
WO 2012143241 A1, which is incorporated herein by reference, discloses inter alia an aramid cord having two plies with a first twist of 250 to 500 t/m and a final twist of 150 to 300 t/m. Aramid is a relatively costly material. In addition, the number of turns is very high, increasing the manufacturing costs.